1. Field of the Invention
The present invention relates to an underwater granulating method and an underwater granulating die in which thermoplastic resin material is kneaded, melted and extruded by a screw type kneading extruder so as to be granulated under water, and particularly relates to the improvement of the manner in which it starts.
2. Related Art
Generally, an underwater granulating method using a screw type kneading extruder and an underwater granulating apparatus are widely used to granulate a large quantity of thermoplastic resin material, such as polyethylene, polypropylene, etc. from raw powder into pellets. That is, raw powder is supplied to a screw type kneading extruder where it is kneaded and melted, and then extruded as thin strings from nozzles of a die provided on the head of the kneading extruder. The strings are extruded into cooling water at a temperature range of 40 to 80.degree. C. circulating in an underwater granulating apparatus, and at the same time cut by a cutter, so as to be cooled by the cooling water and granulated into granular pellets.
In such an underwater granulating method, there sometimes arises a case where the apparatus must be stopped temporarily due to problems associated with the apparatus, the raw material system or the product system, or due to routine maintenance requirements. In that case, the screw type kneading extruder and the underwater granulating apparatus are stopped temporarily and restarted after a short time after the problem or maintenance has been attended to.
Heretofore, such temporary stopping and restarting of the screw type kneading extruder and the underwater granulating apparatus are performed in the following manner. First, the supply of the raw powder is stopped, and at the same time the screw type kneading extruder is stopped. However, the heating of the resin material remaining in the inside of the extruder is continued so that it remains melted. Next, the cutter is separated away from the die surface and stopped, and the cooling water is exhausted from the underwater granulating apparatus to ensure that the melted resin material in the nozzles is not cooled and solidified by the cooling water. The die surface is kept at a high temperature of about 250.degree. C. without contacting the cooling water, so that the die is kept in a restart condition. In the case of restart, first, the cutter device is driven. Next, steps of returning the cutter to the die surface, supplying and circulating cooling water to the underwater granulating apparatus, and driving the screw type kneading extruder so as to extrude the melted resin material from the die are performed substantially at the same time.
In addition, in the case where the screw type kneading extruder is driven for the first time, or in the case of routine inspection, overhaul or the like is performed (i.e., in the case where driving is performed in the state where no resin material is included) first, an underwater granulating apparatus 3 is separated from the extruder as shown in FIG. 4, so that only the screw type kneading extruder is driven and the melted resin material 12 flows incontinently from nozzles 7 of a die 2. Next, when the melted resin material flowing incontinently is brought into a normal stationary state, the drive of the screw type kneading extruder is stopped while the heating state is maintained, the surface of the die 2 is cleaned, and the underwater granulating apparatus 3 is coupled with the die 2 so as to be brought into a stand-by state. Thereafter, start-up is performed in the same manner as in the above-mentioned case of temporary stoppage.
With temporary stoppage and restart performed in such a manner as mentioned above, a conventional underwater granulating method of thermoplastic resin material has had the following problems.
Because the die surface is kept at a high temperature to keep the resin material in the melted state during stoppage, the resin material contained in the nozzles melts and flows over the die surface. Further, because the screw type kneading extruder is also maintained in the heated state in running, part of the melted resin material filling the upstream portion of the die flows through the nozzles. As a result, a space is produced in the upstream portion in the die, and the air enters the space from the nozzles. As a result, the resin material melted at a high temperature reacts with oxygen in the air and is oxidized, thus deteriorating its quality. Therefore, pre-running of the screw type kneading extruder for eliminating the deteriorated portion of the resin material is performed before start as shown in FIG. 4, so that the resin material extruded from the nozzles is wasted. When the manufacturing facility is 20 ton per hour, the quantity of the wasted materials reaches 300 to 500 kg in two to three minutes running at half speed or lower. In addition, in the case of the stopping conditions where the screw type kneading extruder can run, the screw type kneading extruder is operated temporarily with a minimum number of rotations so as to extrude melted resin material from the nozzles to avoid a space being produced in the upstream portion of the die even in the stop state of the extruder. Also in this case, the extruded resin material is wasted. In either case, because melted resin material is extruded from the nozzles in a period from stop to restart, the underwater granulating apparatus is separated from the die as shown in FIG. 4.
To start the screw type kneading extruder, first, the die surface where the melted resin material flows inconsistently is cleaned out after completion of the pre-running. Next, the underwater granulating apparatus is coupled with the die disposed on the head of the screw type kneading extruder, and the cutter device is driven. Next, the steps of moving the cutter to approach the die surface, supplying and circulating the cooling water to the underwater granulating apparatus, and driving the screw type kneading extruder so as to extrude the melted resin material from the die are performed at substantially the same time. Such a series of starting operations must be performed rapidly before the melted resin material flows from the nozzles. In addition, in these starting operations, the timing relation between supplying and circulating the cooling water to the underwater granulating apparatus and extruding the melted resin material from the die is important. If the cooling water is supplied and circulated too early, the die surface is cooled suddenly so that a difference of temperature arises locally to thereby produce an unevenness in extruding. If the melted resin material is extruded too early, the melted resin material adheres to the cutter, so that granulating is not possible. When the timing is off, the above operations from the pre-running are repeated again thus wasting more resin material and manpower.
Further, wear-resistant material is applied on the die surface to thereby form a hardened layer on the die surface in order to protect against contact by the cutter. Maintained at a high temperature of about 250.degree. C., the hardened layer contacts the cooling water of 40 to 80.degree. C. so as to be cooled suddenly so that this hardened layer receives a thermal impact. Cracks are generated in the hardened layer by repetition of this cooling. This may be one of the causes of separation of the hardened layer.